The present invention relates to a method for treating waste water containing neutral salts comprising monovalent ions, and especially, the method for treating waste water containing neutral salts comprising monovalent ions wherein the monovalent ions contained in industrial waste water are recycled by being separated and recovered as acids and bases which composed the neutral salts, thereby eliminating the neutral salts from the treated waste water released to the environment.
Liquid effluent containing acids, bases, and their neutral salts is often generated from industrial processes using acids, bases, or both. Generally, a method for treating industrial liquid effluent is used wherein a volume of the liquid effluent is reduced by evaporation and the condensed water is discharged. However, currently, membrane separating techniques such as reverse osmosis, ultrafiltration, electrodialysis, and electrolytic dialysis are being developed.
JP-A-60-24439 (1985) discloses a method for treating radioactive liquid waste containing nitrate salts, wherein low level radioactive liquid waste containing nitrate salts is supplied to a central compartment of an electrolytic cell comprising three compartments separated by two separators. One separator is a cation exchange membrane made of fluorocarbonpolymer and the other separator is an anion exchange membrane. The nitrate salts are decomposed by electrolytic dialysis so as to generate nitric acid in an anodic compartment, and alkali hydroxide or ammonium hydroxide in a cathodic compartment, while the radioactive materials are concentrated in the central compartment.
FIG. 3 is a flow chart showing the method disclosed in JP-A-60-24439 (1985). Referring to FIG. 3, 1 indicates the anodic compartment, 2 is the cathodic compartment, 3 is the central compartment, 4 is an anolyte tank, 5 is a catholyte tank, 11 is the anion exchange membrane, and 12 is the cation exchange membrane.
In the electrolytic dialysis cell disclosed in JP-A-60-24439 (1985), an ion exchange membrane made of fluorocarbonpolymer is used as the cation exchange membrane 12 dividing the central compartment 3 and the cathodic compartment 2.
The ion exchange membrane has superior durability in alkaline aqueous solution. On the other hand, the anion exchange membrane 11 dividing the central compartment 3 and the anodic compartment 1 is a strongly basic anion exchange membrane or a weakly basic ion exchange membrane. In accordance with electrolytic dialysis, the concentration of nitrate salts in the stream in the central compartment decreases, and the concentration of radioactive elements which are scarcely permeable through the cation exchange membrane such as uranium, plutonium, Am-241, Ce-144, Ru-106, Nb-95, Zr-95, and the like increases.
The method disclosed in JP-A-60-24439 (1985) comprising the steps of decomposing nitrate salts in low level radioactive liquid waste by electrolytic dialysis using ion exchange membranes, concentrating radioactivity in the liquid waste, and treating the concentrated radioactive liquid waste at a processing facility for a medium or high level radioactive liquid waste, is regarded as an industrially usable method which enables recovered nitric acid and hydroxides to be recycled. In accordance with embodiment 1, for example, of the method disclosed in JP-A-60-24439 (1985), sodium nitrate concentration in the untreated liquid waste decreases from 2.4 mole/liter to 2.0 mole/liter in the treated liquid waste. However, the concentration of the salts in the treated liquid waste are preferably decreased more.
FIG. 4 refers to FIG. 7.14, p. 358 of Derek Pletcher and Frank C. Walsh, Industrial Electrochemistry, 2nd Edition. According to FIG. 4, the electrodialysis cell is an electrolytic cell composed of units comprising a diluted stream compartment and a concentrated stream compartment separated by a cation exchange membrane and an anion exchange membrane. The cell is used to transfer and condense salts from the diluted stream, i.e. the supplied stream, to the concentrated stream, and thereby, desairing the diluted stream. In FIG. 4, 13 indicates the cation exchange membrane, 14 is the anion exchange membrane, 15 is the anodic compartment, 16 is the cathodic compartment, 17 is an entry for the diluted stream, 18 is an exit for the diluted stream, 19 is an entry for the concentrated stream, 20 is an exit for the concentrated stream, 21 is an entry for the anolyte, 22 is an exit for the anolyte, 23 is an entry for the catholyte, and 24 is an exit for the catholyte. The anion in the diluted stream permeates through the anion exchange membrane 14 along with the positive potential gradient and is transferred into the concentrated stream. The cation in the diluted stream permeates through the cation exchange membrane 13 along with the negative potential gradient and is transferred into the concentrated stream. As such, the anion and the cation are neutralized to form a salt. Water is electrolyzed to generate oxygen and the anolyte becomes acidic in the anodic compartment 15, and water is electrolyzed to generate hydrogen and the catholyte becomes basic in the cathodic compartment 16. The above explained electrodialysis cell is used industrially for desalting water containing salt to obtain fresh water, or for recovering salt.
Sometimes, various divalent or higher valent ions exist as impurities in common industrial liquid effluent containing salts of monovalent ions. If the divalent or higher valent ions are cations, the ions permeate the cation exchange membrane 13, and if the ions are anions, the ions permeate the anion exchange membrane 14. Alkali metal ions and ammonium ions are generally stable under the chemical conditions pertinent to liquid effluents, and the above mentioned ions always produce water soluble compounds. However, almost all hydroxides of divalent and higher valent cations are insoluble in water, except for divalent ions such as Mn, Ni, Co, Zn, Cd, Cu, and the like which form complexes that dissolve in an aqueous ammonia solution. Therefore, when the divalent or higher valent cations permeate the cation exchange membrane and enter into the basic catholyte, the cations precipitate and operation of the electrolytic dialysis cell becomes difficult. Even if ammonium hydroxide is used as a base in the catholyte to generate water soluble hydroxides, impurities are contained in the recovered ammonium hydroxide, and cause problems such as the restricted use of the recovered ammonium hydroxide. Some divalent and higher valent cations easily deposit as metals onto the cathode, and make operation of the electrolytic dialysis cell difficult. On the other hand, as for divalent and higher valent anions, anions of elements containing oxygen such as As, Sn, Ge, V, Mo, W, Se, Te, and the like exist. However, carbonic acid, boric acid, and phosphoric acid exit in industrial liquid effluent containing salts of monovalent ions. If the above anions permeate through the anion exchange membrane and enter the anolyte, the anions are contained in the recovered acid of the monovalent ion, and restrict the use of the recovered acid.
Furthermore, when lowering the concentration of neutral salts of monovalent ions in the liquid effluent, using electric current to eliminate parasitic ions, which are not necessarily to be eliminated, lowers the electric current efficiency.
An object of the present invention is to provide a method for eliminating salts in waste water and for recovering acids and bases without impurities, which has not been achieved by the prior art relating to electrolytic dialysis of liquid effluent containing salts of monovalent ions. The issues to be solved are as follows:
1. Lowering the concentration of salts of monovalent ions in waste water. PA1 2. Eliminating impurities other than monovalent ions in the recovered acids and bases. PA1 3. Increasing the concentration of the recovered acids and bases as much as possible. PA1 4. Decreasing the consumption of electric power necessary for processing waste water and recovering the acids and bases. PA1 E.sub.cell : loading electric potential (Volts) PA1 N: the number of ion exchange membrane pairs PA1 I: electric current (Ampere) PA1 R.sub.ca : electric resistivity of cation exchange membrane (ohm) PA1 R.sub.an : electric resistivity of anion exchange membrane (ohm) PA1 R.sub.di : electric resistivity of diluted stream (ohm) PA1 R.sub.co : electric resistivity of concentrated stream (ohm)
In order to achieve the above objectives, waste water to be processed is supplied first not directly to an electrolytic dialysis cell, but to a diluted stream compartment in a prepositive electrodialysis cell. A concentrated stream in the prepositive electrodialysis cell containing salts generated by neutralization of ions permeated through a pair of ion exchange membranes is supplied to an electrolytic dialysis cell, which is composed of units. Each of these units comprises three compartments, i.e. an anodic compartment, a central compartment, and a cathodic compartment, which are separated by a cation exchange membrane and an anion exchange membrane respectively, for decomposing the salts to corresponding acids and bases, wherein the acids and bases are recovered.
The diluted stream in the prepositive electrodialysis cell is supplied to a main electrodialysis cell in a subsequent step in order to eliminate residual salts until a final target concentration is achieved.
In order to achieve the above objectives, an ion exchange membrane which is selectively and preferentially permeable only to monovalent ions (such that ions other than monovalent ions hardly permeate the membrane) is used in the present invention. In accordance with selecting an ion exchange membrane which is selectively and preferentially permeable only to monovalent ions for the ion exchange membrane composing the prepositive electrodialysis cell, the concentration of ions other than the monovalent ions in the stream supplied to the electrolytic dialysis cell decreases. Accordingly, it becomes possible to decrease the concentration of cations other than the monovalent ions in the basic catholyte of the electrolytic dialysis cell, and to decrease the concentration of anions other than the monovalent ions in the acidic anolyte.
The concentration of salts of the monovalent ions in the diluted stream coming out from the prepositive electrodialysis cell is lower than the concentration in the waste water to be treated. However, the concentration of ions (other than the monovalent ions) relative to the concentration of the monovalent ions is higher than the concentration in the waste water to be treated. The diluted stream is supplied to the diluted stream compartment of the main electrodialysis cell at a first stage of the plural main electrodialysis cells which are combined in series to eliminate the ions to a final target concentration. If the ions to be eliminated for environmental requirements are only monovalent ions, a cation exchange membrane, an anion exchange membrane, or both of the membranes are selectively and preferentially used which only permeate monovalent ions. If all ions regardless of valency must be eliminated for environmental requirements, an ion exchange membrane is used which permeates all of the cations and anions effectively.
A membrane which permeates only the monovalent ions selectively and preferentially can also be used as the ion exchange membrane for decreasing the concentration of ions other than monovalent ions in the catholyte or anolyte of the electrolytic dialysis cell.
Treated waste water, in which the concentration of monovalent ions has been decreased, which is released from the central compartment of the electrolytic dialysis cell, can be mixed with waste water to be treated and can again be supplied to the prepositive electrodialysis cell, because the stream has a lower concentration of ions, except for the monovalent ions, than that of the waste water to be treated.
The concentrated stream in the main electrodialysis cell at the first step has a higher concentration of ions, except monovalent ions, compared to the concentration of monovalent ions than that of waste water to be treated depending on the situation. Therefore, it is not preferable to mix the concentrated stream with waste water to be treated and to supply it to the diluted stream .compartment of the prepositive electrodialysis cell without any processing.
In the main electrodialysis cell, the quantity of ions transferred from the diluted stream compartment to the concentrated stream compartment is proportional to the quantity of electricity transmitted through a pair of ion exchange membranes and the number of membrane pairs. The quantity of electricity is mainly controlled by the specific electric conductance of the diluted stream under a given load voltage to the electrolytic cell. The specific electric conductance decreases approximately in proportion to the decrease in the concentration of salts in the diluted stream. On the other hand, a back diffusion phenomenon becomes noticeable when the concentration ratio of diluted stream to concentrated stream, that is the concentration gradient, increases. Specifically, the efficiency of the electric current decreases when the electric current density at the surface of the ion exchange membrane decreases.
In the first stage of the main electrodialysis cell for eliminating ions, the ratio of diluted stream concentration to concentrated stream concentration (concentration gradient) becomes very large when it is necessary to decrease significantly high ion concentrations to significantly low concentrations by a batch process wherein the treated stream in a diluted stream circulating tank is circulated between the tank and the electrodialysis cell until the ion concentration in the treated stream reaches a designated concentration. Moreover, a the specific electric conductivity of the diluted stream decreases remarkably in accordance with the decrease in ion concentration. In an industrial application of the electrodialysis cell, it is preferable to restrict the chargeable voltage to, for instance, approximately 50 volts for safety. Accordingly, when the specific electric conductivity decreases, the quantity of electric current (current density) also decreases.
According to the present invention, a process for the treatment of waste water that reduces the final ion concentration to a designated low value is achieved by providing a method wherein the ion concentration is not decreased by a batch process in a restricted stage of the electrodialysis cells, but by a process using all stages of a series of combined electrodialysis cells with a similar ratio at each of the stages.
FIG. 1 is a flow chart showing a preferred embodiment of the present invention.
A typical feature in the composition of the present invention compared to the prior art, which only has electrodialysis cells, is that the composition of the present invention is provided with a recovery block comprising a combination of an electrolytic dialysis cell and half of a prepositive electrodialysis cell, and a treatment block comprising half of a prepositive electrodialysis cell and a succession of plural main electrodialysis cells combined in series (in 3 stages for instance).
The waste water to be treated is supplied to the prepositive electrodialysis cell provided at an interconnecting location between the recovery block and the treatment block. In the prepositive electrodialysis cell and each of the main electrodialysis cells combined in series to the prepositive electrodialysis cell, the concentration of salts preferably decreases at a constant ratio. For instance, when the final salt concentration in the waste water to be treated must be decreased by 1/256 by a combination of a prepositive electrodialysis cell and three main electrodialysis cells combined in series, reducing the salt concentration by 1/4 at each of the stages achieves the objective. If the final salt concentration must be reduced by 1/1296, reducing the salt concentration by 1/6 at each of the stages achieves the objective. If the final concentration to be achieved is 1/4096, reducing the salt concentration by 1/8 at each stages is necessary. If the number of the main electrodialysis cells combined in series increases to four, reducing the salt concentration by 1/4.2 at each of the stages is sufficient to reduce the salt concentration in the waste water to be treated by 1/1296.
The relationship between the loading electric potential to the main electrodialysis cell and the current through the electrodialysis cell under a condition wherein the concentrated stream is commonly used as an anolyte and as a catholyte can be expressed by the following equation: EQU E.sub.cell =NI(R.sub.ca +R.sub.an)+NIR.sub.di +(N+I)IR.sub.co (Eq. 1)
where,
If the loading electric potential over the electrodialysis cell is a constant value, the quantity of electricity available to the electrodialysis cell is significantly controlled by the electric resistivity of the diluted stream. As the specific electric resistivity of the diluted stream is approximately proportional to the salt concentration in the diluted stream, the quantity of electricity is approximately proportional to the salt concentration in the diluted stream.
Accordingly, a substantial part of the electric power consumed in the main electrodialysis cell generates heat in the diluted stream compartment, and it is necessary to cool and remove the heat from the circulating stream in the electrodialysis cell because the concentrated stream is also heated through the ion exchange membrane. The specific electric resistivity of the electrolyte aqueous solution decreases by about 3% per temperature elevation of 1.degree. C., and the specific electric resistivity of the ion exchange membrane has the same characteristics. Therefore, effective operation of the electrodialysis cell can be achieved by properly controlling the temperature of the diluted stream.
The molar quantity of salts transferred from the diluted stream, which flows through the electrodialysis cell at a constant flow rate, to the concentrated stream is proportional to the salt concentration in the diluted stream at a constant potential. Accordingly, the number of necessary ion exchange membranes at each stage of the series of electrodialysis (the number of sets of diluted stream compartments and concentrated stream compartments) is approximately same even if the molar quantity of salt to be transferred varies significantly.
On the other hand, the concentrated stream in the main electrodialysis cells combined in series flows countercurrent to the diluted stream, and the concentrated stream accumulates salt which is transferred from the diluted stream. When the quantity of concentrated stream is less than the quantity of diluted stream by the same ratio as the concentration decreasing ratio of the diluted stream in the prepositive electrodialysis cell, the final concentration of the concentrated stream becomes approximately equal to the initial concentration in the waste water to be treated, and the concentrated stream is preferable for use in the process by recycling. In the above case, the ratio of salt concentration in the diluted stream and the concentrated stream in the main electrodialysis cell finally equals approximately the square of the decreasing rate of salt concentration in the diluted stream.
The concentrated stream contacting the anode in the prepositive electrodialysis cell and the main electrodialysis cell becomes acidic, and the concentrated stream contacting the cathode becomes basic. Therefore, in the conventional desalination of sea water, anolyte is kept acidic by adding acid in order to prevent the precipitation of magnesium hydroxide in the anolyte. In the present invention, the concentrated stream circulates commonly through all anode compartments and cathode compartments including the concentrated stream compartment of the prepositive electrodialysis cell and each of the main electrodialysis cells respectively. However, in order to prevent the catholyte from becoming too basic, the concentrated stream which becomes acidic in the anode compartment is supplied to the cathode compartment and is circulated.
The water used to replace the water supplied to the concentrated stream at the last stage of the series of main electrodialysis cells and the water supplied to the concentrated stream in the prepositive electrodialysis cell is preferably pure water in order to prevent contamination of the concentrated stream with impurities.
The ratio of salt concentration in waste water to be treated to salt concentration in the diluted stream in the prepositive electrodialysis cell is preferably equal to the ratio in the main electrodialysis cell. Therefore, the ratio of salt concentration in the concentrated stream to the salt concentration in the diluted stream equals to n(n-1) by making the ratio of the flow rate of the diluted stream to the flow rate of the concentrated stream equal to the ratio (n) of the salt concentration in the feed water to be treated to the salt concentration in the diluted stream, and the ratio of the salt concentration in the concentrated stream to the salt concentration in the feed water to be treated equals to (n-1).
The electrolytic dialysis cell comprises plural pairs of an anode compartment having an anion exchange membrane as a separating membrane, a cathode compartment having a cation exchange membrane as a separating membrane, and a central compartment having an anion exchange membrane and a cation exchange membrane as separating membranes, all of which are arranged alternatively. The concentrated stream from the prepositive electrodialysis cell is supplied to the central compartments as a common feed stream, and concurrently, an equal quantity of stream equal to the difference between the quantity of supplied stream and the quantity of stream permeating through the ion exchange membranes, flows out from the central compartments.
The acid is generated in the anolyte which circulates to the anode compartment, and the base is generated in the catholyte which circulates to the cathode compartment. The concentration of the acid and the base depends on the sum of the quantity of absorbing stream supplied to the anolyte and the catholyte and the quantity of stream permeating through the respective type of ion exchange membranes. According to the present invention, the acid and the base can be recovered relatively easily with a high concentration because salt concentration in the feeding stream to the electrolytic dialysis cell is several times as high as in the salt concentration in the waste water to be treated. However, an upper limit of salt concentration is generally restricted by chemical resistance to the acid or base of the respective ion exchange membrane. Although the acid and the base recovered by the present invention are both highly concentrated and highly pure, the acid and the base can be concentrated and purified easily by a conventional method if their usage requires.
The circulating stream in the central compartment of the electrolytic dialysis cell has preferably the same salt concentration as the salt concentration in the waste water to be treated because the excess circulating stream returns to the stream which is to be fed to the prepositive electrodialysis cell for re-treating.
The ion exchange membranes used in the present invention preferably have a selective transport number of at least 0.98 for either total cations or total anions. Further, the ion exchange membranes for selectively transporting monovalent ions in the present invention preferably have a selective transport number of at most 0.5 for ions other than the monovalent ions.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.